Polyamide resins, which have superior physical and chemical properties, have been used as raw materials in a wide range of applications, for example in production of various molded products, films, fibers, and others. Particularly in recent wide-spread use of the resins as automobile parts, electric and electronic parts, mechanical parts, and construction materials, there is further increase and diversification of the requirements for the polyamide resins in properties. For example, for use as a gear in automobile electric power-steering systems, which are exposed to high load for an extended period of time, the resin is required to be improved not only in strength and rigidity, but also in toughness and durability. It is also demanded to be superior in lubricity for improvement in abrasion resistance and also in dimensional accuracy for keeping resistance of gear parts constant and also to have low specific density.
It is well known that a reinforcing material such as glass fiber or inorganic filler is blended into a polyamide resin for improvement in strength and rigidity, and such a method is used in various application fields (see e.g., Patent Document 1). However, for example, blending of a glass fiber into a polyamide resin composition leads to increase of its specific gravity, and such a resin is not used favorably as a part for use in applications demanding lubricity, because, when the resin is used for a gear part, the glass fiber therein causes abrasion of the counter metal gear during engagement.
On the other hand, polyamide resin compositions obtained by dispersing swellable lamellar silicate such as synthetic fluorinated mica or montmorillonite in a polyamide resin are attracting attention recently. These resin compositions are known to be superior in strength, rigidity, heat resistance etc., and, for example, a resin composition containing nylon 6 and a synthetic fluorinated mica (see Patent Document 2) and a resin composition containing nylon 6 and montmorillonite (see Patent Document 3) are known. However, there was a need for further improvement in performance, specifically in toughness and durability, of these resin compositions.
In addition, monomer cast nylons (MC nylons) have been known as polyamide resins superior in strength, impact resistance and durability that are produced without use of a reinforcing material such as glass fiber (see Patent Documents 4 and 5). However, the MC nylons are materials unsuitable for injection molding and thus, need in practice machining processing of the block for forming a desired shape, causing a problem in production cost.    Patent Document 1: Japanese Patent application Laid-Open No. Sho 57-137762    Patent Document 2: Japanese Patent Publication No. 2941159    Patent Document 3: Japanese Examined Patent Publication No. 8-22946    Patent Document 4: Japanese Patent application Laid-Open No. 2002-88150    Patent Document 5: Japanese Patent application Laid-Open No. 2005-240973